1. Field of the Invention
The present invention relates generally to air cleaning systems for treating contaminated air or emissions, and more particularly, to a method for sizing a source capture system for an air cleaning system that captures contaminated air from an industrial process or the like.
2. Description of Related Art
In the field of industrial processes, and particularly mechanical industrial processes such as machining, metal working, material fabrication and the like, it is well known to use some type of contaminated air cleaning system to capture and treat the contaminated air and other emissions produced by these industrial process. During operation of these types of industrial processes, contaminated air containing mist, vapor, and smoke is produced, particularly whenever liquid coolants are used on, or near, the working surface of the operation. In a cutting or grinding operation, for example, liquid coolant is evaporated and thrown by the cutter or grinder into the air space surrounding the work area. Although large coolant droplets and chips of material may fall to the ground, small coolant droplets, mist and vapor and very small pieces of material and dust become suspended in the air space surrounding the work area to form a thick haze of contaminated air. In order to capture this contaminated air for purposes of removal and treatment, some type of source capture system is located near the work area.
Two of the most widely used source capture systems are hood arrangements and containment units. In a hood arrangement, such as shown in U.S. Pat. Nos. 1,676,969, 3,340,788, 3,425,335, 4,284,236 and 4,623,022, a hood having a relatively large opening is placed above or adjacent to the work area of the industrial process and a negative pressure is used to draw the air immediately around the work area into the opening of the hood. In a containment unit, such as shown in U.S. Pat. Nos. 4,902,315, 5,023,116, 5,133,246 and 5,133,690, an entire enclosure that is substantially air-tight is built around the industrial process and the contaminated air and emissions are then withdrawn from the containment unit, usually through duct work connected to an opening in the top of the containment unit, often with makeup air being introduced into the enclosure.
Many early systems simply vented the contaminated air and other emissions captured from an industrial process directly to the outside. With the advent of air pollution regulations, source capture systems were connected to air cleaning apparatus in an effort to remove harmful contaminates from the air stream prior to venting it to the outside. Over the years, many different types of air cleaning apparatus have been used in an effort to clean up the contaminated air and other emissions from industrial processes. Typically, contaminated air would be captured at multiple locations or work areas within a factory, for example, and then routed by ventilation duct work to a common air cleaning apparatus, often located outside the factory. A variety of different air cleaning and/or mist removal apparatus, such as helical separators, cyclone separators, scrubbers, mechanical and electrical filters and the like, have been used to clean the contaminated air once it is within the air treatment system. Examples of these type of apparatus are shown in U.S. Pat. Nos. 4,038,056, 4,216,003, 4,364,754, 4,382,807, 4,460,386, 4,350,504, 5,024,681, 5,110,331, 5,120,335, 5,129,930 and 5,178,656, as well as the Hell-Flow.RTM. system manufactured by the assignee of the present invention, the Monsanto Organic Package System manufactured by Monsanto Enviro-Chem Systems, Inc., St. Louis, Mo. and the Duct Type Mist Separator KNA manufactured by Delbag Luftfilter, Germany. The basic source capture systems that bring the contaminated air to these air cleaning apparatus, however, have not changed.
One approach to modifying existing hood-type source capture systems is described in U.S. Pat. Nos. 4,050,368 and 4,125,062 issued to Eakes. In both of these patents, a portion of the exhaust air captured by the hood arrangement is recirculated through the air space above the work area prior to being treated by some type of air cleaning apparatus so that the volume of air per unit time passing through the air cleaning apparatus is reduced. In a preferred embodiment, the hood arrangement has an air current at the front of the work area to further direct the contaminated air into the source capture system. This approach can decrease the amount of makeup air which would otherwise be drawn from the environment surrounding the work area, but it does not otherwise change the fundamental manner in which the contaminated air is drawn into the air treatment system.
While an increasing amount of effort has been focused on improving the effectiveness of air cleaning apparatus for air treatment systems, the basic source capture systems that bring the contaminated air into these air treatment system have not changed. One reason why there has been so little effort focused on improving source capture systems is because of a general assumption about the way in which air moves within the air treatment system. Because contaminated air is captured at a location far removed from the air cleaning apparatus there is an assumption that the manner in which the air is captured will not have a significant effect on the ability of the air cleaning apparatus to properly clean the contaminated air. As will be shown, however, there are many situations in which this assumption does not hold true. Consequently, it would be desirable to provide for a source capture system which takes into account the manner in which air moves within the overall air treatment system so as to increase the effectiveness of the air cleaning apparatus connected to the source capture system.